Geochemistry of the andesite flank lavas of three composite cones within the Atitlán Cauldron, Guatemala

Abstract

Three composite cones have grown on the southern edge of the previously existing Atitlan Cauldron, along the active volcanic axis of Guatemala. Lavas exposed on the flanks of these cones are generally calc-alkaline andesites, but their chemical compositions vary widely. Atitlan, the largest and most southerly of the three cones, has recently erupted mainly pyroclastic basaltic andesites, while the flanks of San Pedro and Toliman are mantled by more silicic lava flows. On Toliman, 74 different lava units have been mapped, forming the basis for sequential sampling. Rocks of all three cones are consistently higher in K2O, Rb, Ba and REE than other Guatemalan andesites. Atitlan’s rocks and late lavas from Toliman have high Al2O3 content, compared to similar andesites from other nearby cones. All major and trace element data on the rocks are shown to be consistent with crystal fractionation involving phases observed in the rocks. If such models are correct, significant differences in the relative proportions of fractionation phases are necessary to explain the varied compositions, in particular higher Al2O3 rocks have fractionated less plagioclase. We speculate that inhibition of plagioclase fractionation could occur in chambers where PH2O is greater and when repose intervals are shorter. The distribution of volcanic vents throughout Guatemala which show this postulated «inhibition of plagioclase fractionation» is systematic with such vents lying just to the south of the main axis. The andesites of the three cones cannot be simply related to the late-Pleistocene rhyolites which are apparently associated with cauldron formation, because unlike the andesites, the rhyolites have markedly depleted heavy REE abundances. Recent dacitic lavas from vents south of San Pedro volcano and silicic pyroclastic rocks which mantle the slopes the San Pedro may reflect residual post-cauldron rhyolitic volcanism.